1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a laser printer. More particularly, the present invention relates to an image forming apparatus having a transfer belt that conveys a developer image formed on a photoconductor to and transfers the developer image onto a recording medium.
2. Description of the Related Art
Color image forming apparatuses are typically classified as either a multi-path type apparatus or a single path type apparatus. A multi-path type apparatus rotates a single photoconductor several times to form a desired color image, and a single path type apparatus rotates a plurality of photoconductors one time to form a desired color image.
Multi-path type image forming apparatuses have both advantages and disadvantages. On the one hand, since a multi-path type image forming apparatus forms a required color image by revolving a single photoconductor several times, it produces color image more slowly than a single path type image forming apparatus. On the other hand, a multi-path type image forming apparatus uses a single photoconductor, and therefore has a reduced number of parts so that it has a simpler construction and is more compact.
FIG. 1 shows an example of a multi-path type color image forming apparatus.
As shown in FIG. 1, the multi-path type color image forming apparatus 1 has a transfer belt 20. The transfer belt 20 puts a plurality of single color developer images, for example, yellow, magenta, cyan, and black developer images, formed at predetermined time intervals on a photoconductor 10, together to form a primary transfer image, and then transfers the primary transfer image to an image receiving medium P.
To enhance transfer efficiency, the transfer belt 20 is generally made of a polymer having a volume resistance of 108 Ω·cm˜1011 Ω·cm. A high resistance coating layer is formed on an outer surface of the transfer belt 20 to prevent image spreading. The high resistance coating layer has a volume resistance higher than 108 Ω·cm˜1011 Ω·cm.
The transfer belt 20 is supported by a driving roller 21 and a driven roller 23 so that it rotates along an endless loop. The driving roller 21 is formed of a metal roller 21a, which may be made of a metal such as aluminum. To drive the transfer belt 20 stably, a rubber layer 21b is formed on the outer surface of the metal roller 21a. The driving roller 21 is grounded by a ground 22 so as to discharge an electric potential which is generated by rubbing of the rubber layer 21b against the transfer belt 20. The electric potential on the driving roller has a polarity opposite to that of used developer. The driven roller 23 is formed of a metal roller 23a, which may be made of a metal such as aluminum, so that a belt cleaning member 28 may firmly contact the transfer belt 20.
An electrostatic latent image which corresponds to a first color, such as yellow, is formed on the surface of the photoconductor 10 by laser beams emitted from a laser scanning unit 19 according to an image signal. The electrostatic latent image is developed into a yellow developer image by a corresponding yellow developing unit 11.
The yellow developer image formed on the surface of the photoconductor 10 is transferred to the transfer belt 20 with pressure and a first transfer-bias voltage which are applied to the transfer belt 20 by a first transfer roller 25.
In the same manner, the remaining color developer images, such as magenta, cyan and black developer images, are individually formed on the surface of the photoconductor 10 by corresponding magenta, cyan and black developing units 13, 15 and 17, and then transferred and superimposed on the yellow developer image on the transfer belt 20 by the pressure and the first transfer-bias voltage of the first transfer roller 25. As a result, a primary transfer image in which the yellow, magenta, cyan and black developer image are superimposed is formed on the transfer belt 20.
The primary transfer image formed on the transfer belt 20 is then transferred to an image receiving medium P with pressure and a second transfer-bias voltage which are applied to the image receiving medium P by a second transfer roller 27. As a result, a secondary transfer image is formed on the image receiving medium P.
After the primary transfer image is transferred to the image receiving medium P, used developer remaining on the transfer belt 20 is cleaned and removed by a belt cleaning member 28. The belt cleaning member 28 is disposed below the driven roller 23 and is placed into contact with or separated from the transfer belt 20 by an actuating member (not shown). The belt cleaning member 28 may be formed of a blade made of, for example, urethane rubber and may have a thickness of approximately 2 mm.
The secondary transfer image transferred to the image receiving medium P is fused onto the image receiving medium P by heat from a heating roller (not shown) and pressure from a compression roller (not shown) of a fusing unit (not shown) while passing by the fusing unit. The image receiving medium P with the fused secondary transfer image is discharged from the image forming apparatus by a discharge roller (not shown) of a discharging unit (not shown).
With the conventional image forming apparatus 1 constructed as described above, a problem occurs in that friction between the image receiving medium P and the transfer belt 20, friction between the transfer belt 20 and the driving roller 21 or the driven roller 23, and the like, generate an electric potential on the transfer belt 20. The electric potential has a polarity opposite to that of the used developer. Thus, when the transfer belt 20 with a transfer image passes by the belt cleaning member 28, used developer on the belt cleaning member 28 is transferred back to the transfer belt 20 due to the opposite polarities, thereby deteriorating the quality of the primary transfer image.
To address this problem, the transfer belt 20 is preferably configured to have a low electric resistance so that the transfer belt 20 is not charged with an electrical potential with a polarity opposite to that of the used developer even though it is rubbed against the driving roller 21 and the driven roller 23. Since the transfer belt 20 is made of a polymer having a high electric resistance layer coated on an outer surface thereof to prevent image spreading during transferring, however, there is a limit to reducing the electric resistance of the transfer belt 20 so as to prevent the pollution by the used developer on the belt cleaning member 28.
Accordingly, to prevent pollution by the used developer on the belt cleaning member 28, the conventional image forming apparatus 1 is configured so that the belt cleaning member 28 contacts the transfer belt 20 at an angle of less than 90° as shown in FIG. 1, or at an angle of approximately 90°. When the belt cleaning member 28 is designed to contact the transfer belt 20 at an angle of less than 90°, and the belt cleaning member 28 is separated from the transfer belt 20, it is positioned almost parallel to the transfer belt 20 so that used developer cleaned by and adhered to the belt cleaning member 28 can fall into and be completely collected by a storage container (not shown). Also, when the belt cleaning member 28 contacts the transfer belt 20 at an angle of approximately 90°, the belt cleaning member 28 is separated from the transfer belt 20 so that it is spaced apart from the transfer belt 20 with a sufficient distance d so that the used developer cleaned by and adhering to the belt cleaning member 28 is positioned sufficiently remote from the transfer belt 20 that it is not attracted to the transfer belt 20. These conditions, however, not only restrict freedom in design, but also increase the space required for the belt cleaning member 181, thereby imposing restrictions on the size of the apparatus.
Accordingly, there is a need for an improved image forming apparatus that prevents used developer from transferring from a belt cleaning member to a transfer belt.